1. Field of the Invention
The field of the invention is a system for preventing an excessive slip of a drive wheel of a vehicle, including a drive wheel velocity detector means for detecting the velocity of the drive wheel, a vehicle velocity detector means for detecting a vehicle velocity, a drive wheel slip value calculating means for calculating a slip value of the drive wheel on the basis of the drive wheel velocity and the vehicle velocity, and a drive wheel torque decrement calculating means for calculating a drive wheel torque decrement in accordance with the slipping state of the drive wheel.
2. Description of the Prior Art
Systems of such type are conventionally known from, for example, Japanese Patent Application Laid-open No. 149250/88.
In such a known system, a PID control is carried out by use of, as a target drive wheel velocity, a drive wheel velocity corresponding to a predetermined slip rate (for example, of 10%) determined on the basis of a vehicle velocity. When the PID control is effected at a constant gain, a gain of a rapid feedback response, i.e., a relatively large gain is set so as to increase the performance of control in a region of a high frequency of use. Moreover, as a region of high frequency of use, a low speed stage of a speed reduction gear is generally selected.
However, various disturbances are encountered during a feedback control of the drive wheel velocity. Particularly when a gear shift is conducted during travelling of a vehicle on a road surface of a low friction coefficient, or when the condition of the road surface is changed from a snow-laden road to a frozen road, the drive wheel velocity may greatly exceed the target velocity in some cases due to a disturbing factor during the PID feedback control, and the resulting large velocity deviation may become a trigger to generate a control hunting.